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Evidence of two local structures in liquid water. Credit: Nature Physics (2026). DOI: 10.1038/s41567-026-03301-8
A study published in Nature Physics provides new molecular-level evidence from simulations that liquid water is not a single uniform substance, but a constantly shifting mixture of two distinct microscopic structures.
The idea that water might exist in two distinct structural states is not new. For decades, scientists have theorized that liquid water is composed of two interconvertible local structures—one denser and more disordered, the other less dense and more ordered.
This "two-state model" has been invoked to explain water's many anomalous properties, including why it becomes easier to compress as it cools and why it reaches maximum density at 4°C (39°F) rather than at its freezing point. But the model has remained controversial because direct molecular-level evidence for the two structures has been elusive.
Phys.org spoke to corresponding author Prof. Xiao Cheng Zeng from City University of Hong Kong about the study and its findings.
"I have been very interested in the topic of phase transitions since I was a graduate student," said Zeng. "I started theoretical research on freezing of liquids when I was a postdoc, but I was always hoping to study freezing of water one day. Since then, I have been particularly interested in the topic of liquid-liquid transition in water."
Yet despite decades of theoretical work, direct molecular-level evidence for the two structures has remained out of reach.
Rotational perspective of a 3D plot of probability distributions P(ρ local , PCI, PCII) of water local structures at 1,800 bar/188 K. Credit: Nature Physics (2026). DOI: 10.1038/s41567-026-03301-8
Molecular-level proof
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